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Supernova
A supernova is a rare astronomical event that occurs during the last stellar evolutionary stages of a massive star's life, whose dramatic and catastrophic destruction is marked by one final titanic explosion. For a short time, this causes the sudden appearance of a 'new' bright star, before slowly fading from sight over several weeks or months. Only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been telescopically seen in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Star of 1604 (SN 1604), but remnants of two more recent supernovae have been found retrospectively. Statistical observations of supernovae in other galaxies suggest they should occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable in modern astronomical equipment. Supernovae are more energetic than novae. In Latin, Nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas (abbreviated SN, plural SNe after "supernovae"). During maximum brightness, the total equivalent radiant energies produced by supernovae may briefly outshine an entire output of a typical galaxy and emit energies equal to that created over the lifetime of any solar-like star. Such extreme catastrophes may also expel much, if not all, of its stellar material away from the star, at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with higher mass elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars. A great proportion of primary cosmic rays comes from supernovae, and they are also potentially strong galactic sources of gravitational waves. Theoretical studies of many supernovae show they can be triggered by one of two basic mechanisms. This can be by sudden re-ignition of nuclear fusion in a degenerate star; or by the sudden gravitational collapse of the massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature to then trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy that can create a supernova explosion. Observations do show there are more uniqueness or complexities to these two simple alternatives mechanisms, but the astrophysical collapse mechanics have been accepted and established by most astronomers for some time. Due to the significant number of diverse astrophysical consequences of these supernovae events, astronomers now deem supernovae studies, across both stellar and galactic evolution, as most important among their many lines of study.